Dielectric bead design for broadband coaxial lines



July 11, 1961 w. E. SLUSHER 2,992,407

DIELECTRIC BEAD DESIGN FOR BROADBAND COAXIAL LINES Filed May 26, 1959 INVENTOR. WILLIAM E. SLUSHER 1;. $4.4... ATTORNEY United States Patent 2,992,407 DIELECTRIC BEAD DESIGN FOR BROADBAND COAXIAL LINES William E. Slusher, West Newton, Mass., assignor, by

mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed May 26, 1959, Ser. No. 816,027 1 Claim. (Cl. 333-96) This invention relates to coaxial transmission lines and more particularly to so-called bead supported coaxial lines.

.The inner conductor of a coaxial line must be properly arranged or supported coaxially of the outer conductor. For low losses, at very high operating frequencies, an air dielectric is desirable rather than a solid dielectric. It is common practice in using coaxial lines designed for high frequency operation with low losses, to provide spaced dielectric beads for coaxially supporting the inner conductor and in such arrangements the major portion of the dielectric is air. However, the presence of the beads causes wave reflections at the interfaces between the air dielectric and the bead dielectric.

These reflections in head supported coaxial cables have been reduced or eliminated in two principal Ways. First, the heads have been arranged in spaced pairs whereby reflections from one bead is cancelled by reflections from the other bead. Secondly, individual beads have been compensated by undercutting the inner conductor, but this has been done simply by taking into account the nominal impedance of the bead, and without taking into consideration the capacitance introduced by the sharp corners of the cuts which are formed in the metallic conductors. Consequently, certain deficiencies in the prior methods of obtaining reflectionless transmission are apparent. It has been determined that the spaced pairs are frequency sensitive depending upon the thickness of the bead used, and neglect of the thickness results in a voltage standing wave ratio beyond that expected from the compensation. It has been found also that undercut beads which were designed without regard to the capacitance resulting from the sharp corners did not perform as expected, especially in the high frequency ranges of 3000 megacycles per second and above.

Accordingly, it is the principal object of the present invention to provide a bead structure design utilizing the aforementioned undercutting principle and yet cancelling the undesirable capacitance introduced by the discontinuous inner conductor.

The above and other objects of the invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a perspective view of a portion of a coaxial line with a portion removed to show the bead arrangement of the inner conductor; and

FIG. 2. is a longitudinal sectional view showing details of the coaxial line of FIG. 1.

Referring to FIG. 1, there is shown generally a coaxial cable at having an inner conductor 12 of non-uniform circular cross section and a tubular outer conductor 14 of uniform cross section. The inner conductor 12 is positioned concentrically within outer conductor 14 and is spaced therefrom by insulating beads 16 which have their outer peripheries contacting the inner surface of the outer conductor 14. Any suitable type of insulating material may be used for the beads 16, one having a low dielectric constant being preferred. They may be made in two pieces, if desired, for convenience in putting the beads on, or the beads may be made in one piece, or might be molded onto the conductor, or the conductor may be made in sections, to fit together at the point where the bead occurs, as by screwing one section into the other. In FIG. 2, the inner conductor 12 is shown provided with spaced undercuts, that is, reduced diameter portions 18, each of which form shoulders 20 and 20 which are perpendicular or normal to the axis of the conductor 12. The undercut portions 18 will have a depth which will depend on the dielectric constant of the selected material of which the head is formed in order to provide a characteristic impedance at each bead. This characteristic impedance will be equal to the characteristic impedance of the transmission line in the air space portions S, in FIG. 1, between the beads. Each bead 16 is formed with collars 22 which are received by the undercut portions 18 and the bead has an outer periphery 24 which contacts the inner wall of the outer conductor 14. The outer diameter of each collar 22 is made to equal that of the inner conductor 12 while its inner diameter is substantially equal to that of the undercut or reduced diameter portions 18.

By way of further explanation, in FIG. 2, collars 22 are designated as A and C portions, the axial length of bead 16 is designated at B and the space between C and A portions of successive collars designated at D.

The A and C portions provide high characteristic impedances that add series inductive components to the coaxial line. It can be shown that any discontinuity in the inner conductor, as at the shoulders 20 and 20 and having axial symmetry, may be represented by lumped capacitances thereat. The inductive components provided by the A and C portions will cancel the capacitances set up by the discontinuities in the inner conductor if physical dimensions of the A and C portions be accurately determined. The length of B therefore becomes unimportant, any convenient length sutficing.

Since, as aforementioned, the inner and outer diameters of collars 22, or the A and C portions of FIG. 2, are made substantially equal to the undercut diameter 18 and the diameter of the inner conductor 12 at those portions between successive collars, represented as portion D, it will be necessary only to determine the proper lengths of the A and C portions.

The characteristic impedances of the A and C portions are calculated, which involve the use of the wellknown impedance chart, and the inductance per unit length determined relative to the characteristic impedance at these portions, Ihe capacitances produced by the discontinuities 20 and 20 of inner conductor 12 are calculated, averaged, and the length of both A and C portions are then determined by calculating the amount of inductance required to cancel this average capacitance.

Having thus described the invention, what is claimed is:

A coaxial transmission line comprising a tubular outer conductor having an inner wall providing a continuous surface and an inner conductor extending axially of the outer conductor, said inner conductor having reduced diameter portions spaced therealong each of which is defined by a pair of opposed shoulders extending generally normal to the longitudinal axis of the inner conductor, dielectric spacing elements having an axial bore for the reception of the reduced diameter portions of the inner conductor, each spacing element having a large diameter portion in contact with the inner wall of the outer conductor and opposed axial collars of equal length extending therefrom, one of said axial collars having its outer end surface disposed in abutting relation with one of said opposed shoulders and the other of the axial collars having its outer end surface disposed in abutting relation vw'th the other of said opposed shoulders, said axial collars having a common diameter equal to the diameter of the opposed shoulders on the inner conductor to provide a smooth continuous outer surface therewith and a length to provide inductance sufficient to compensate for 3 V the capacitance introduced by the shoulders wherebythe impedance of the coaxial lines over a broadband frequency range remains substantially constant.

Peterson June 6, 1933 Young Feb. 10, 1948 4 Salisbury Mar.- 9, :19'48 Weber Dec. 23, 1952 Lintzel Dec. 18, 1956 FOREIGN PATENTS Italy Nov. 17, 1949 Germany Oct. 1, 1953 Germany Oct. 8, 1953. 

